Find duplicate e-mail --C language version + MySQL version

(LeetCode: database) find duplicate e-mail

Topic Description: Writing a SQL query to find all duplicate Person table e-mail.

+----+---------+
| Id | Email   |
+----+---------+
| 1  | a@b.com |
| 2  | c@d.com |
| 3  | a@b.com |
+----+---------+

Based on the above inputs, your query should return the following results:

+---------+
| Email   |
+---------+
| a@b.com |
+---------+

Note: All e-mail all lowercase letters.

MySQL programs:

SELECT
    email
FROM
    person
GROUP BY
    email
HAVING
    count(email) > 1;

If you are using MySQL can easily solve this problem, and now to solve this problem in C language.

C language program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void main()
{
	int len;
	char *str[] = {
		"[email protected]",
		"[email protected]",
		"[email protected]"
	};
	len = sizeof(str) / sizeof(*str);//求字符串元素个数
	int i, j, k, l;
	char *r[10] = { NULL };//字符串标记，存放重复的字符串
	l = 0;//r的下标
	for (i = 0; i < len; i++)
	{
		for (j = i + 1; j < strlen(*str); ++j)
		{
			if (str[i] == str[j])
			{
				for (k = 0; k < l; k++)
				{
					if (!(strcmp(str[i] , r[k])))
					{
						break;
					}
				}
				if (k == l) 
					r[l++] = str[i];
			}
		}
	}
	for (int i = 0; r[i]!=NULL; i++)
		printf("%s\n", r[i]);
}